Presently, the ethylene.cndot..alpha.-olefin-nonconjugated diene copolymer (hereinafter referred sometimes to as EPDM because propylene is commonly used as the .alpha.-olefin) is excellent in processability, weather resistance, ozone resistance, heat resistance and the like, so that it is widely used in fields such as automobile parts, industrial parts, building materials and the like. It is so far difficult, however, to make EPDM possess characteristics of further high strength and hardness without damaging its features as described above.
For example, for obtaining vulcanized rubbers having a hardness of 45 or more (Shore D hardness according to ASTM D 2240), the following methods are known, but any of them has problems.
For example, when the polymer component of the vulcanizable rubber composition is a diene rubber such as natural rubber, styrene-butadiene rubber, acrylonitrile-butadiene rubber or the like, vulcanized rubbers having the above hardness can be obtained by increasing the amount of a vulcanizing agent. In this case, however, there is a defect that the weather resistance and heat resistance of the resulting vulcanized rubber are inferior.
On the other hand, when the polymer component is a rubber having a small content of unsaturated hydrocarbon such as EPDM, vulcanized rubbers having the above hardness cannot be obtained by merely increasing the amount of the vulcanizing agent. Therefore, when EPDM is used as the polymer component, there is employed for example a method of using EPDM having a large content of ethylene as well as blending a large amount of a reinforcing agent (e.g. carbon black) and small amounts of a softening agent and a plasticizer, or a method of adding a thermosetting alkylphenolic resin to the EPDM described above.
In the former method, however, there are problems that the resulting vulcanizable rubber composition increases in Mooney viscosity so largely that its kneading with a Banbury mixer and its processing with a roll are difficult, and also that vulcanized rubbers obtained from it are short of hardness. On the other hand, the latter method is superior to the former one in processability at the time of production of the vulcanizable rubber composition. However, it has a defect as described below: The curing reaction of the alkylphenolic resin at the time of vulcanization produces water which in turn induces foaming of the rubber to make it difficult to obtain a satisfactory product, so that this method cannot be applied to normal-pressure continuous vulcanization, and therefore this method can only be applied to vulcanization under pressure such as autoclave vulcanization, press vulcanization and the like of poor productivity. Further, vulcanized rubbers obtained by such the method have a problem that they are inferior in a speed at which they recover their original form after deformation, i.e. snappiness.
Also, there is a method of raising the hardness by blending EPDM with a polyolefin resin (e.g. polypropylene) or polystyrene resin. However, when the polyolefin resin is blended, there is a problem that the Mooney viscosity of the resulting vulcanizable rubber composition increases so remarkably that particularly processability on rolls and at the time of extrusion is inferior. Further, when the polystyrene resin is blended, there is a problem that the strength of the resulting vulcanized rubber is low.